CONTAINER FOR ELECTROMAGNETIC COOKING AND PRODUCTION THEREOF

Abstract: A bowlike container has a structure, the whole or part of which includes a single layer of a solid titanium alloy, or multiple layers of one or more titanium alloys. The resistivity of the titanium alloy(s) is in a range between 80x10⁻⁶Ω.cm and 200x10⁻⁶Ω.cm. The container is formed into a desired shape by hot forging or superplastic forming of one or more sheets of titanium alloy at 500°C or above. After a long time of use, the container hardly gets fatigued and keeps its excellent effect while enduring sufficiently for may years. Accordingly, practical effects are extremely great.

Description: The present invention relates to a method of manufacturing a heating container for an electromagnetic cooking device, in particular, relates to making an electromagnetic cooking device container, comprising titanium alloy, by hot forming or superplastic forming. Recently, electromagnetic cooking devices using electromagnetic induction as an energy source have been widely utilized for both business and home appliances instead of gas cooking stoves. A heating principle of such an electromagnetic cooling device is such that an induction coil is provided underneath a plate on which cooking containers such as pans, pots, and cooking skillets are placed, an alternating magnetic field generated by such induction coil produces eddy current in the cooking container, then the eddy current flow in the container results in generating heat depending on a specific impedance of material constituting the cooking container. As described above, it has been conventionally considered that generation of the eddy current in the cooking container by means of the alternating magnetic field requires a magnetic material such as stainless steel, steel or the like (see, Nikkei New Materials, dated March 18, 1991, p.20) for that material of the container. Therefore, the containers for electromagnetic cooking devices such as pans, pots, and cooking skillets have generally been made of iron, or made of clad metal which is superposed with two or more different kinds of metals, for example, stainless steel and aluminum alloy etc. However in the container made of materials as in the above, the inventors of the present invention has found problems in that, where iron is used, the repeated heating and cooling in the extremely strong magnetic field disadvantageously provides deformation in the container due to produced magnetostriction, and in addition, tends to cause rust, and is more weighty. In a container made of clad metal, repeated heating and cooking through use produces different thermal expansion and magnetostriction between the different kinds of metals which are superposed each other. This results in more weight in addition to peeling on bonding of those different kinds of metals. Furthermore, unitary bonding of different kinds of metal requires higher costs for scrapping process in addition to a complicated and therefore expensive manufacturing process. A drawback is that austenite stainless steel arranged on the inside surface of the container contacts with meat or vegetable or the like on cooking so this tends to scorch and stick to the inside surface of the container. JP-A-01 107 716 discloses a titanium cooking container to be heated by electromagnetic induction. An object of the invention is to provide a method of manufacturing a container of an electromagnetic cooking device which is easy to handle, light weight and exhibits a high heating-efficiency due to electromagnetic induction and a graded corrosion-resistance without peeling on bonding surface between the different kinds of metals and without deformation even in the repeat use. According to the present invention, a method of manufacturing a method of manufacturing a container of an electromagnetic cooking device which comprises either a single titanium alloy layer, or two or more laminated layers of one or more kinds titanium alloy, at least a part of the container forming a vessel shape, from an outside surface to an inside surface, in which the specific resistance value of the or each titanium alloys is equal to or more than 80 x 10 -6 Ω·cm and equal to or less than 200 x 10 -6 Ω·cm, the method comprising the steps of forming one sheet of titanium alloy plate or a titanium alloy plate superposed of two or more sheets of titanium alloy plates into a desired shape at a molding temperature equal to or more than 500°C by means of hot forming or superplasticity molding, and oxidizing the titanium alloy plate. As a result of research and study, it has been found that composition of titanium alloy is adjusted to produce a suitably larger specific resistance value and to enable heat generation by electromagnetic induction as in magnetic materials in spite of non-magnetic material, and that a surface (cooking surface) is covered by its oxide to prevent scorch and stick to the inside of the container on cooking. It is impossible to treat the titanium alloy by means of ordinary cold treatment because the titanium alloy has a considerably higher strength when placed at a room temperature. However, the titanium alloy is found to be formed easily into a desired shape according to the hot forming otherwise the superplastic forming which utilizes superplastic phenomenon that can be seen in titanium alloys. The present invention has been made in accordance with such knowledge and intention, and is constructed in that, the container comprises a single titanium alloy layer, or two or more laminated layers of one kind titanium alloy or two or more kinds of titanium alloys, on the most part of the container forming a vessel shape, and each specific resistance value of the titanium alloys is equal to or more than 80 x 10-6 Q-cm and equal to or less than 200 x 10-6 Q-cm. The present invention is constructed so that the container comprises a single titanium alloy layer, or two or more laminated layers of one kind titanium alloy or two or more kinds of titanium alloys, on a part of the container forming a vessel shape, from an outside surface to an inside surface, and each specific resistance value of the titanium alloys is' equal to or more than 80 x 10-6 Q-cm and equal to or less than 200 x 10-6 Q-cm. All of the specific resistance values of titanium alloys described in this specification are the values at an ordinary temperature (room temperature). Generally, it is known that the specific resistance value varies depending on temperature. Therefore, it is understood that, when the specific resistance of the titanium alloys shows a value within the range defined in accordance with the present invention at a room temperature, such specific resistance value is deemed to be included in the present invention even if the specific resistance departs, at a temperature other than the room temperature, from the scope defined in accordance with the present invention. The reason why a specific resistance value equal to or more than 80 x 10-6 Q-cm is given to the titanium alloy is in that, although it is difficult to heat pure titanium (Ti) having a specific resistance of an extent of 50 x 10-6 Q-cm by means of electromagnetic induction, here when converting into an alloy by adding aluminum and the like on pure titanium, the specific resistance value becomes larger, thus obtained higher specific resistance value becomes equal to or more than 80 x 10-6 Q-cm, then a sufficient heating (calorific) value for cooking is obtained by the electromagnetic induction. The reason why a specific resistance value equal to or less than 200 x 10-6 Q-cm is given to the titanium alloy is that an upper limit of the specific resistance value of the titanium alloy easily available in the industrial field at the present time is equal to 200 x 10-6 Q-cm. However in the container of the electromagnetic cooking device, basically, a larger specific resistance value provides a greater heating value. A forming temperature thereof is made equal to or more than 500°C because a strength (that is, a flow stress) of the titanium alloy in such temperature region becomes smaller, hence, shapes of the pan or cooking skillet or wok or the like are easily formed by the hot forming such as hot forming or by superplastic forming preferably under the condition of vacuum or inert atmosphere. In the case where a container comprises a plurality of laminated titanium alloy layers, a temperature region equal to or more than 500°C is provided as, at such temperatures, those alloy layers are joined into an unitary form by solid phase diffusion at the same time of forming. In the superplastic forming, a thin plate member is formed into a vessel shape. In the light of this, a superplastic blow forming method may be preferable wherein the thin plate is forced on the inner circumference of a mold or die by gas pressure to produce a desired shape. A gas having oxidizing property used, for imposing a forming pressure to produce an oxide film on the inside surface (cooking surface) of the container at the time of forming. In this case, when oxygen is employed as an oxidizing gas, a partial pressure of the oxygen gas may be optionally selected, however preferably an extent of 0.001 to 0.1 atmosphere. Instead of employing an oxidizing gas for forming the oxide film on the container, by dipping the container for which forming was completed into a suitable oxidizing solution or by anodizing the container in a suitable solution, such an oxide film can be also formed thereon. In case where a whole container or part thereof comprises two or more laminated layers of one kind of titanium alloy or two or more kinds of titanium alloys, then the number of layers or each thickness of the layer are optional, any combination of a plurality of kinds of titanium alloys having different compositions are also optional. Of course, two or more layers of laminated titanium alloy layers having the same composition can be also used. According to the invention, titanium alloy having a higher specific strength which may give graded corrosion-resistance is employed as a material of the electromagnetic cooking device container, occurrence of rust is suppressed even in long term use. A considerably light weight container is realized in comparison with the conventional container in case of forming a container having the same strength and the same shape and dimension as the conventional container. Therefore in the whole container or a part of the container, for example, when a professional cooker uses the container made of titanium alloy for a long time, the cooker receives an extremely lower degree of fatigue and is easy handled due to a light-weight of the container. Since titanium alloy is of non-magnetic property, no distortion due to magnetostriction arises even in an intensive magnetic field, any deformation is prevented in the container using the titanium alloy. In addition to the above, when a whole container or a part of the container comprises a structure formed of two or more titanium laminated alloy layers, each titanium alloy layer is in solid phase state and is formed completely into one unit, and even with different compositions, the same titanium is used as a basic material. This prevents almost all thermal expansion difference, and eliminates peeling on bonding surface of each layer. Because of a high specific-resistance of titanium alloy, a depth of skin effect of eddy current generated in the alternating magnetic field (a largeness of current is equal to 1/e (= 0.368) of the largeness in the surface, and proportional to a square of 1/2 of the specific resistance) comes deeper (this means a smaller skin effect and therefore a deep penetrated depth of the eddy current) and the container can be effectively heated by the electromagnetic induction. A specific resistance value equal to or more than 80x 10-6 Q-cm of each titanium alloy layer provides a sufficient heating value for cooking due to electromagnetic induction. The titanium alloy formed on an inside surface of the container enables uniform heating on the entirety of the container depending on the magnetic field and prevents scorch and stick to the container (because the scorch and stick arise by partial heating). The coating of titanium oxide film on the inside surface of the container allows greasy and oily components to remain in the oxide film, and this completely eliminates the scorch and stick. A hot forming or superplastic forming at an temperature equal to or more than 500 C at which there is only a small variation in the strength of titanium alloy gives easy forming of the plate materials made of titanium alloy into various desired shapes such as for pans, cooking skillets, and woks. At the time of forming, the titanium alloy layers of two or more laminated layers are securely joined with each other by solid phase diffusion. The container having a lamination structure with titanium alloy layers facilitates scrapping process because of lamination of the titanium alloys each other even with difference compositions being employed. BEST MODE FOR CARRYING OUT THE INVENTION A feature of the present invention is described with respect to a first embodiment and a second embodiment and a comparison example, as follows. (First Embodiment) Each container of an electromagnetic cooking devices produced in the first embodiment comprises only titanium alloy layers. Table 1 shows each layer structure, the material of each layer, the thicknesses thereof, presence of an oxide film on the cooking surface, and a manufacturing method, in respect of 11 examples of containers (No. 1 to No. 11). Examples 5, 9 and 11 are produced in accordance with the first embodiment the present invention. As shown in Table 1, each container of Nos. 1 and 2 was made of only single titanium alloy layer produced by hot forming, each container of Nos. 3, 4, and 5 was made of only single titanium alloy layer produced by superplastic forming, each container of Nos. 6 and 7 was made of two layers of titanium alloy layers produced by hot forming, and each container of Nos. 8, 9, 10 and 11 was made of three layers of titanium alloy layers bonded and formed by SPF (a process for solid phase bonding at the same time of superplastic forming) (in Nos. 6 to 11, each of the first layers is the most outside layer, that is, has an outside surface of the container). The specific resistance value of each titanium alloy layer of those layers was equal to or more than 80 x 10-6 Q-cm at an ordinary temperature. In concrete values, the specific resistance value of Ti-8Al-lMo-lV was equal to 199 x 10-6 Q-cm, that of Ti-2Al was 82 x 10-6 Q-cm, that of Ti-6Al-4V was 171 x 10-6 Q-cm, that of Ti-3Al-2.5V was 126 x 10-6 Q-cm, that of Ti-5Al-5Sn-2Zr-2Mo-Si was 171 x 10-6 Q-cm, and that of Ti-8Mn was 92 x 10-5 Q-cm, where Al, Mo, V, Sn, Zr, Si and Mn represent aluminum, molybdenum, vanadium, tin, zirconium, silicon, and manganese, respectively. Each of the containers Nos. 1, 2, 6, and 7 was produced by placing the titanium alloy plate of each layer in Table 1 on a forming machine and press forming at a forming temperature shown in Table 1 under argon (Ar) gas atmosphere. In each of the containers Nos. 6 and 7, the laminated titanium alloy layers have been incorporated with each other because of solid phase diffusion bonding of the titanium alloy layers with each other being performed at the same time of the press forming. In each of the containers Nos. 3, 4, 5, 8, 9, 10, and 11, for superplastic forming, there was employed a superplastic blow forming method in which a forming pressure is produced by gas pressure. In each of the containers Nos. 8, 9, 10, and 11, the titanium alloy layers have been incorporated with each other because of solid phase diffusion of the titanium alloy layers with each other being performed at the same time of the superplastic forming. All the forming temperatures were equal to or more than 500°C, setting thereof was suitably executed depending on the kinds of the titanium alloys which were used. In concrete, the forming temperatures were 750 C for each of the containers Nos. 1, 2, and 6, 900 C for each of the containers Nos. 3, 4, and 5, 500 C for each of the container No. 7, 880°C for each of the containers Nos. 8 and 9, and 920°C for each of the containers Nos. 10 and 11. For each of the containers Nos. 5, 9, and 11, by using a gas mixture containing oxygen gas as a forming pressure gas for superplastic forming, an oxide film was formed on the inner surface (cooking surface) of the container at the same time of the forming. A partial pressure of the oxygen gas at that time was of 0.01 atmosphere in each of the containers Nos. 5 and 11, and 0.05 atmosphere in the container No. 9. (Second Embodiment) In a container of an electromagnetic cooking device produced in the second embodiment, the bottom of the containers (a pan bottom or a pot bottom) comprises a structure of a titanium alloy layer, the side-surface (a pan wall or a pot wall) of the container is made of stainless steel, and the pan bottom and the pan wall are joined and unitarily formed. Table 2 shows for the pan bottoms of containers (Nos. 12 to 14), and shows each layer structure, material and thickness of each layer, presence of oxide film on the cooking surface, and a manufacturing method therefor. Example 14 is made in accordance with the present invention. As shown in the Table, each pan bottom of containers Nos. 12 and 13 was hot forged and made of only single titanium alloy layer, and the pan bottom of the container No. 14 was made of two layers of titanium alloy produced by superplastic forming (in No. 14, the first layer is the most outside layer, that is, an outside surface). Each pan bottom of the containers Nos. 12 and 13 was produced by placing the titanium alloy plate shown ln Table 2 on the forming machine and press forming it at the forming temperature shown in Table 2 under argon (Ar) gas atmosphere. In the container No. 14, a pan bottom was produced by a superplastic blow forming in which forming pressure was given by gas pressure. Then, an oxide film was formed on the inside surface (cooking surface) of the pan bottom by using a gas mixture containing oxygen gas as a forming pressure gas, at the same time of the forming. Partial pressure of the oxygen gas was equal to 0.1 atmosphere. The forming temperature for each pan bottom of the containers Nos. 12 and 13 was 750 C, and that for the pan bottom of the container No. 14 was 900°C. (Comparison Example) For further comparison, four examples of containers Nos. 15 to 18 are produced, where each container for an electromagnetic cooking device produced in each comparison example has the same layer structure in the entirety thereof. Table 3 shows a material, a surface treatment on a cooking surface, and remarks, for each comparison example. As shown in the Table, the container No. 15 is made of only pure Ti, the container No. 16 is made of only iron (Fe) similarly to a conventional container, the container No. 17 is made of a conventional clad material which is superposed with a stainless steel (SUS 430) as the most outside layer (outside surface of the container) and an aluminum (Al) alloy on the inside thereof, and the container No. 18 is made of a conventional clad material having a stainless steel (SUS 304L) further arranged on the inside of the container No. 17. Each specific resistance of the materials in the above described comparison example is as follows. The specific resistance of the Ti was (47 to 55) x 10-6 Q-cm, that of the Fe was 9.8 x 10-6 Q-cm, that of SUS430 was 60 x 10-6 Q-cm, that of the Al alloy is 5.8 x 10-6 Q-cm, and that of SUS304L was 72 x 10-6 Q-cm. To prevent the scorch and stick to the container, a surface treatment, i.e., a known fluorocarbon resin treatment, was carried out on the cooking surface of only the container No. 17, and in the container No. 16, the cooking surface was sufficiently familiarized with cooking oil, instead of the surface treatment. (Evaluation) Using 18 container examples of Nos. 1 to 18, 200 omelet cooking were performed by an electromagnetic cooking device with 2 kW capacity. Thereafter, an evaluation is performed for heating state, damage of the container, deformation of the container, extent of fatigue of the cooker due to the weight, corrosion resistance, and scorch and stick. The result of the evaluation is shown in Table 4. No Heating State Damage Deformation Degree of Fatigue Corrosion Resistance Scorch and Stick Fist Embodiment 1 (○) (○) (○) (○) (○) ○ 2 (○) (○) (○) (○) (○) ○ 3 (○) (○) (○) (○) (○) ○ 4 (○) (○) (○) (○) (○) ○ 5 (○) (○) (○) (○) (○) (○) 6 (○) (○) (○) ○ (○) ○ 7 (○) (○) (○) (○) (○) ○ 8 (○) (○) (○) ○ (○) ○ 9 (○) (○) (○) ○ (○) (○) 10 (○) (○) (○) (○) (○) ○ 11 (○) (○) (○) (○) (○) (○) Second Embodiment 12 (○) ○ ○ ○ (○) ○ 13 (○) ○ ○ ○ (○) ○ 14 (○) ○ ○ ○ (○) (○) Comparison Example 15 x - - - - - 16 (○) ○ x x ○ (○) 17 (○) x ▵ x (○) (○) 18 (○) x x x (○) ▵ In the Table, (○) mark designates that 200 cookings were performed without problem and yet thereafter continuous cooking was also available. ○ mark designates that 200 cookings were performed with few problems and thereafter continuous cooking was available. ▵ mark designates that 200 cookings were achieved, but thereafter repeat cooking was unavailable, and x mark designates that cooking is difficult from the first or a failure due to peeling or the like arises in cooking function before reaching the 200th cooking. As apparent from Table 4, for heating state and corrosion resistance, each example of Nos. 1 to 14 could be used for cooking exceeding 200 times without problem. For damage and deformation, each of the examples Nos. 1 to 11 could be used even in cooking exceeding 200 times without problem, each of the examples Nos. 12 to 14 provided some problems in 200 times of cooking but thereafter continuous cooking was available, For fatigue, each of the examples Nos. 1, 2, 3, 4, 5, 7, 10, and 11 could be used without problem even in cooking exceeding 200 times, and each of the examples Nos. 6, 8, 9, 12, 13, and 14 could be used in cooking exceeding 200 times but some troubles arose because of the examples having a little thicker thickness. For scorch and stick, each of the examples Nos. 1 to 14 was good, in particular, each of the containers Nos. 5, 9, 11, and 14 having an oxide film were very good. In Nos. 12, 13, and 14, with titanium alloy layers only on their pan bottoms, scorch and stick were prevented by the titanium alloy layers on the heating portions (pan bottoms) which was conventionally apt to scorch and stick, and therefore satisfactory results were obtained. On the contrary, a container, for which all the evaluation items were substantially satisfied, was not found in each of the comparison examples Nos. 15 to 18. In particular, it was impossible to perform an induction heating by using the container No. 15 because of its lower specific resistance, i.e., (47 to 55) x 10-6 Q-cm. Consequently, the container No. 15 could not be used from the first. The container No. 16 had a larger deformation due to magnetostriction, and the containers Nos. 17 and 18 were considerably damaged because of deformation or peeling between the layers, thus, any of the containers Nos. 16-18 were not practical. The present invention is not limited in compositions of the titanium alloys, and in combinations of the number of layers or in combinations of the kinds of alloys in cases of laminated construction with two or more layers of titanium alloy. Where only of a part of the container has a structure of titanium alloy layer, as in the second embodiment, the pan bottom comprises a titanium alloy layer, the pan wall comprises stainless steel, and the pan bottom and the pan wall are joined and incorporated with each other. However the present invention is not limited to this. The present invention requires that a structure comprising a titanium alloy layer are used for a part of the container, for example, the bottom is made of stainless steel and the pan walls comprises titanium alloy layer and the like. However, the position on which a titanium alloy layer is to be used, and the material to be used for positions other than those on which a titanium alloy layer is used, are not limited. According to the container of the electromagnetic cooking device of the present invention, in effect, the use of titanium alloy having a specific resistance value equal to or more than 80 x 10-6 Q-cm as a material of the containers, provides graded corrosion-resistance, prevents scorch and stick, deformation due to magnetostriction and peeling on bonding surface of laminated layers with each other, in addition, obtains sufficient amount of heat for cooking by electromagnetic induction, and provides light weight. The titanium alloys are easily obtained in the market as long as its specific resistance value is equal to or less than 200 x 10-6 Q-cm. In advantages, the cooker hardly suffers fatigue even if cooking by using the container of the invention for a long time. Moreover, the container exhibits satisfactory effects which can withstand long years of practical use, therefore, such effects on practical use are extremely large. Since the method of manufacturing an electromagnetic cooking device according to the present invention, utilizes a hot forming or a superplastic forming at forming temperature equal to or more than 500 C in order to easily mold the container into a desired shape, the container having an upgraded characteristic can be readily formed. Particularly, the use of the superplastic forming is capable of forming with ease even for a thin-thickness large-drawing shape which is hardly formed by the ordinary deep drawing or the like. Industrial Applicability As hereinbefore fully described, the present invention is effectively applicable for manufacturing an electromagnetic cooking device container preventing deformation thereof and peeling on bonded interface between the different kind metals even in repeat use, an electromagnetic cooking device container having a high heating efficiency by electromagnetic induction and a graded corrosion-resistance, and an electromagnetic cooking device container which is easily handled with light weight.

Claims: A method of manufacturing a container of an electromagnetic cooking device which comprises either a single titanium alloy layer, or two or more laminated layers of one or more kinds titanium alloy, at least a part of the container forming a vessel shape, from an outside surface to an inside surface, in which the specific resistance value of the or each titanium alloys is equal to or more than 80 x 10 -6 Ω·cm and equal to or less than 200 x 10 -6 Ω·cm, the method comprising the steps of forming one sheet of titanium alloy plate or a titanium alloy plate superposed of two or more sheets of titanium alloy plates into a desired shape at a molding temperature equal to or more than 500°C by means of hot forming or superplasticity molding, and oxidizing the titanium alloy plate. A method of manufacturing a container of an electromagnetic cooking device as claimed in claim 1 wherein a superplastic blow forming method os used to form the container in which the titanium alloy plate is forced on an inner circumference of a mold or die by gas pressure to produce the desired shape, and in which the said gas has a oxidizing property. A method of manufacturing a container of an electromagnetic cooking device according to claim 2, in which the said gas is oxygen gas having a partial pressure in the range of 0.001 to 0.1 atmosphere. A method of manufacturing a container of an electromagnetic cooking device according to claim 1, in which the step of oxidizing the titanium alloy plate is carried out by dipping a desired portion of the formed container into a suitable oxidizing solution. A method of manufacturing a container of an electromagnetic cooking device according to claim 1, in which the step of oxidizing the titanium alloy plate is carried out by anodizing a desired portion of the formed container in a suitable solution.

Inventors: KATAGIRI TATSUO, OKATO NOBUYOSHI, OSADA KUNIAKI, SHIRAKAWA KEIJI, KATAGIRI, TATSUO, OKATO, NOBUYOSHI, OSADA, KUNIAKI, SHIRAKAWA, KEIJI, KATAGIRI, TATSUO, TECHN. RES. CTR. RES. AND DEV., OKATO, NOBUYOSHI, TECHN. RES. CTR. RES. AND DEV.
Applicants: NIPPON YAKIN KOGYO CO LTD, NIPPON YAKIN KOGYO CO., LTD.
Application year: 1994
Main classification: A47J36
IPC codes: H05B6, A47J36, A47J27